Inductively coupled plasma mass spectrometry (ICP-MS) is a type of mass spectrometry which is capable of detecting metals and several non-metals at concentrations as low as one part in 1015 (part per quadrillion, ppq) on non-interfered low-background isotopes. This is achieved by ionizing the sample with inductively coupled plasma and then using a mass analyzer to separate and quantify those ions. Compared to atomic absorption techniques, ICP-MS has greater speed, precision, and sensitivity.
The sample may be prepared by a number of methods. One preferred method is laser ablation, allowing for the elemental analysis of accurately defined spatial locations. In this method, a pulsed high power laser is focused on the solid sample and creates localized analyte portions, namely short pulses of ablated material which can be collected and entrained in a gas flow within a collection container and then be swept into the plasma.
Another method of preparation of the sample is the generation of single droplets (or micro-droplets). This may be achieved by piezo-electrically driven dispenser heads, such as described in O. Borovinskaya, B. Hattendorf, M. Tanner, S. Gschwind, D. Günther, “A prototype of a new inductively coupled plasma time-of-flight mass spectrometer providing temporally resolved, multi-element detection of short signals generated by single particles and droplets”, J. Anal. At. Spectrom., 2013, 28, 226-233.
A transfer system is arranged between the laser ablation device or the droplet generator (DG), respectively and the ICP unit. It allows for transferring the pulses of ablated material to the ICP unit. It may comprise a transfer tube and/or a device embedding the particles in a gas jet for the transfer. Further, it comprises a connection to the ICP gas input.
In the ICP unit, the aerosol or droplet is vaporized, atomized and ionized and then transferred to the mass analyzer.
There is a substantial time delay, the Signal Delay, between the trigger pulse triggering the laser ablation or the droplet generation and the appearance of the corresponding ions at the entrance of the mass analyzer. In a conventional LA-ICP-MS or DG-ICP-MS this delay is not compensated or accounted for. The measurement is started and the signal is continuously acquired for some time. This method is not optimal, because the assignment of each signal peak to the corresponding laser pulse or droplet generation has to be made in a post processing step. Integration of each peak signal is also best done in post processing, since real time integration would rely on the laser or trigger clock and the acquisition clock being perfectly synchronized. In practice the two clocks will slightly diverge which will cause Moiré patterns to appear in the data.
Further, in the case of laser ablation, if the sample is moved under the laser during measurement (a so-called “line scan”), the positional information corresponding to each laser shot is only approximate, as calculated from the time after the start of measurement.
The Signal Delay and the post-processing thus introduces further computational burden, makes the processing slower and may affect the precision of the results.